Indeed, at sea, the wave motion causes, among other things, heave, i.e. an oscillating motion of vertical translation of the floating units. When the latter support tools such as drill bits, the heave needs to be compensated for so that the bit is permanently in contact with the hole bottom.
To compensate for such motions, there are three major families of devices:                devices that are set in the drill string,        devices that are interposed between the string and the drill rig lifting system, and        devices that are integrated in the lifting system.        
The type of device of the third family conventionally solves the heave compensation problem by making a first block, referred to as crown block, mobile, and it also comprises a second block, referred to as travelling block. The first block is referred to as crown or stationary block because it is stationary with respect to the mobile unit and the second block is referred to as travelling block because it is mobile with respect to the mobile unit. However, through the agency of the heave compensator, the second block is made substantially stationary with respect to the sea bottom. It is reminded that a block is a mechanical device allowing a load to be lifted by several cable strands.
Furthermore, this type of device generally comprises at least one cylinder connected to accumulators, notably pneumatic accumulators. These accumulators occupy a large volume, which is penalizing, especially for an offshore application.
Documents KR-2012/035,432 and CN-101,654,145 describe examples of compensation systems.
An important quantity for such a device is the variation of the force to be exerted on the first block (crown block) according to the path traveled by the mobile unit in relation to a constant value that it should withstand without heave. The difference between the real force and this constant value is referred to as error.
Furthermore, document FR-2,575,452 (U.S. Pat. No. 5,520,369) describes such a device comprising two blocks, at least one compensation cylinder connected to accumulators, a cable and two articulated arms including sheaves and rods that allow to compensate for a motion for an element hanging from a mobile unit. This system allows to reduce the volume of the accumulators through a suitable geometry of the articulated arms. However, this system is not perfect: an error remains between the real force and the constant value because the cylinder pressure setpoint is not constant since the volume of the accumulators is limited and the ideal gas specific constant γ of the air contained in the accumulators is different from one. Besides, the articulated arm system does not allow all of the above error to be rectified. The remaining error causes a variation in the hanging weight during drilling, which prevents the driller from rigorously applying the weight on bit setpoint.
The present invention relates to a motion compensation system for an element hanging from a mobile unit, the system comprising two blocks, at least two articulated arms, a compensation cylinder and a cable. According to the invention, at least one characteristic length of the articulated system (for example: the length of a link or the distance between two sheaves of two articulated arms) is controlled according to the motion so as to decrease the error and therefore to improve the motion compensation.